The present invention relates to a ship for use in offshore operations which has one or more vertical openings in the hull, so-called moonpools. Openings in the hull of a ship which, e.g., is to be used as a drill ship, are quite common today, occurring both singly and severally, and are used for a variety of tasks. In some situations it is desirable to arrange fixedly mounted equipment in proximity to such a moonpool in order to carry out specific tasks. This could be, e.g., a derrick, a crane arrangement for launching and taking up, for instance, ROV, or other equipment for the execution of operations through these openings in the hull.
The openings are usually arranged along the longitudinal centre line of the ship and may be of a variety of cross-sectional shapes and sizes, but often they are rectangular or approximately rectangular, and if there are several openings, these are arranged one after the other along the longitudinal centre line of the ship and may have the same cross-sectional shape and size. If the openings are not used for specific tasks, they may be built into a ship to have an impact on its motions
The rectangular openings have an effect on the stress distribution in, inter alia, the deck and hull bottom plates when the ship is exposed to stresses caused by waves and other loads. In some areas, notches may occur which result in concentrations of stress, where the level of stress may be considerably higher than the average stresses in the deck or hull bottom plates. Stress concentrations will occur in particular at the corners of rectangular openings, even though the corners are designed so as to minimise notch effect. It is also possible to alleviate the situation by designing bracket plates and local reinforcements appropriately, but the stress concentration factor will often be high, and a stress concentration factor of 2 is by no means unusual.
When designing and constructing a drill ship, it will be necessary to take such local stress concentrations into account, and they form the basis for calculating, inter alia, the fatigue life of a ship. Under certain operating conditions, such as, e.g., long-term operations in the North Sea, fatigue may be a decisive criterion for the dimensioning of a typical transverse section of a drill ship. In practice, this means that the dimensions of the deck and hull bottom plates must be increased considerably to meet the fatigue capacity requirements. In addition to the production process being more complex, this will also involve substantially higher costs. Furthermore, the load picture and the stress distribution will be more complicated and uncertain when, e.g., a derrick for use through the hull opening is to be fixedly mounted in the area of stress concentrations, or in proximity to such areas. When positioning fixed equipment of this kind, it is of course an advantage that stresses and stress concentration should be the lowest and most uniform possible with no local stress concentrations, or the potential for these. This may also result in designs that are simpler to produce, which in turn will help to reduce costs in connection with the production of the ship.
The conventional notion of stress distribution in deck and hull bottom plates is that the stresses in the deck and hull bottom plates will, when the ship is exposed to stresses caused by waves and other loads, increase approximately linearly with the distance from the transverse neutral axis of the ship. In other words, the deck and hull bottom plates which are arranged furthest from the ship""s transverse neutral axis will have the highest stress level. Furthermore, the stresses are intensified in local stress concentrations as a result of the design of the deck and hull bottom plates, if included in this design there are elements which produce notch effect. These elements may be sharp corners, openings, welded-in equipment or the like. Thus, on the basis of these factors, two solutions for the reduction of the stress concentrations become evident. One of these is to reduce the notch effect in the deck and hull bottom plates, and the other solution, towards which the present invention is directed, concentrates on the positioning and design of the deck and hull bottom plates at the site of cut-outs which, e.g., are rectangular, in relation to the transverse neutral axis of the ship.
The invention relates to a ship as disclosed in the preamble of claim 1, having the characteristics that are disclosed in the characterising clause of claim 1. Additional embodiments of the invention are disclosed in the other claims.
If the ship""s deck and hull bottom plates which have cut-outs for moonpools are placed closer to the transverse neutral axis, the average stress level will be lower, and the stress concentrations resulting from notches will be smaller. This in turn will be a major consideration when dimensioning of the deck and hull bottom plates, and especially in relation to dimensioning for dynamic load and fatigue life. On drill ships having moonpools, as covered by the present application, there will at all times be stress concentrations as a result of notches at the points where the openings, or the so-called moonpools, pass through the deck and hull bottom plates. By displacing these deck and hull bottom plates, or optionally just the deck plate or the hull bottom plate towards the transverse neutral axis of the ship, a lower average stress level will be obtained. This will give lower stress peaks and thus help to reduce the need for plate thickness, which in turn results in a less costly ship.
In the proposed arrangement, the deck plates (on the upper deck) and the bottom plates on the side of the moonpool will be continuous without any cut-outs which result in notches and thus stress concentrations.